they feel half of a whole. But allison and amail ya will always be true. Let's go. Reporter: For half of their lives, they were in a way one person, conjoined at the chest and abdomen, identical twins that didn't separate properly in the uterus. One doctor advised them to terminate the pregnancy. But specialists at the children's hospital of philadelphia thought the twins could survive and be separated. We were so worried if the girls were going to be okay. eporter: THEIR TWINS WERE Born into a world of medical uncertainty. They came out screaming. Reporter: Last year, when the girls were just 8 months old, a massive team of surgeons and nurses worked hours to separate them, making sure they would survive. They were completely separated. Two separate girls. The really, the most amazing feeling. Reporter: It's now been over a year, allison and amelia are thriving. They won't remember their life before, their parents, of courses, won't forget. I'm thankful every single day and I can't describe it. To see the girls and see them climb and get into things, aggravated as I get, I can't help but laugh because they're an absolute miracle. Reporter: Her twin challenges are now more ordinary. When people say, are they twins, are they identical or what? I say, you have no idea. It's been a crazy journey this last year. Reporter: For "good morning america," dr. Jennifer ashton, abc news, new york.

This transcript has been automatically generated and may not be 100% accurate.

The Science of Conjoined Twins

A rare look at the science behind the phenomenon of conjoined twins.

Conjoined Twins Survive Separation

Conjoined twins are in serious but stable condition after an 18-hour surgery.

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